The Facebook app had 85.5 million monthly unique users in the U.S. in December across iOS, Android and RIM's mobile platforms, a dramatic increase from the 65 million unique users it had last March, when comScore began tracking mobile app data.

During the same time, Google Maps saw its numbers slide from about 80 million in August and September to slightly more than 70 million in October, when Apple removed Google Maps as an iOS 6 default app. But Google may yet make a comeback with the launch of its new Maps app for iOS6.

“Facebook finished the year strong to capture the #1 position from Google Maps. Of course Apple´s decision to replace Google Maps with Apple Maps on iOS 6 also caused a decline in Google Maps usage in October, which is largely responsible for the position swap,” wrote Andrew Lipsman, VP of industry analysis at comScore, in a blog posting about the latest figures.

“Meanwhile, Google Maps has been clawing its way back the past few months after getting reinstated on iOS 6, so look for the competition for the #1 spot to heat up as we head into 2013.”

ComScore's figures show that Facebook's mobile app accounted for 23% of the total time spent on mobile apps (excluding Instagram´s 3% stake), while all of Google's apps collectively accounted for 10%.

This means users spend more than 1 out of every 3 minutes of their mobile app time on either Facebook or a Google app, ComScore said.

ComScore's figures are based on browsing behavior from a panel of users, along with supplemental app usage data from publishers. The numbers represent active visitors to apps, rather than app downloads, similar to the way comScore tracks visitors to websites.

Although Facebook held the top mobile app spot as of December, Google held the next five spots with Google Maps, Gmail and YouTube, Google Play and Google Search.

Lipsman attributed this to the growth of Android and the overall popularity of Google´s services.

The comScore figures are a welcome boost for Facebook, which has faced Wall Street pressure to generate revenue from its growing mobile audience.

“2013 will present an interesting dynamic as Google and Facebook wrestle for app supremacy, while other media properties look to carve out a niche or establish a more prominent position,” Lipsman said.

“If there is one thing we can all bet on, it´s that the market will look very different a year from now.”